1. Field of the Invention
The present invention relates to a method of producing a zinc oxide thin film, a method of producing a photovoltaic device, and a method of producing a semiconductor device substrate.
2. Description of the Related Art
In a conventional photovoltaic device comprising amorphous silicon hydride, amorphous silicon germanium hydride, amorphous silicon carbide hydride, microcrystalline silicon or polycrystalline silicon, a reflecting layer formed on the back thereof is utilized for improving a collection efficient at long wavelengths. Such a reflecting layer preferably exhibits effective reflection properties at a wavelength near the band ends of a semiconductor material at which absorption is low, i.e., a wavelength of 800 to 1200 nm. Materials satisfying this condition include metals such as gold, silver, copper and aluminum.
Also, an uneven layer optically transparent in the predetermined wavelength range is generally provided as a light confinement layer between the metal layer and a semiconductor layer in order to improve a short-circuit current density Jsc by effectively utilizing reflected light.
Further, in order to prevent deterioration in properties due to a shunt path, a layer comprising a translucent material exhibiting conductivity, i.e., a transparent conductive layer, is provided between the metal layer and the semiconductor layer.
In general, these layers are deposited by a method such as vacuum deposition or sputtering, and show an improvement of 1 mA/cm2 or more in short-circuit current density.
For example, in xe2x80x9cLight Confinement Effect in a-SiGe Solar Cell on 29p-MF-2 Stainless Substratexe2x80x9d, Abstracts of the 51st Science Lecture Meeting of the Applied Physical Society of Japan, p. 747, 1990, xe2x80x9ca-SiC/a-Si/a-SiGe Multi-Bandgap Stacked Solar Cells With Band Gap Profilingxe2x80x9d; Sannomiya et al., Technical Digest of the International PVSEC-5; Kyoto, Japan, p. 387, 1990, and xe2x80x9cP-IA-15a-SiC/a-Si/a-SiGe Multi-Bandgap Stacked Solar Cells With Bandgap Profilingxe2x80x9d, Sannomiya et al., Technical Digest of the International PVSEC-5, Kyoto, Japan, p. 381, 1990, an improvement in short-circuit photoelectric current is achieved by a structure comprising a reflecting layer as a back layer composed of Ag, and a light confinement layer as an uneven front layer composed of zinc oxide.
Also, T. Tiedje, et al., Proc. 16th IEEE Photovoltaic Specialist Conf. (1982), p. 1423 and H. Deckman, et al., Proc. 16th IEEE Photovoltaic Specialist Conf. (1982), p. 1425 disclose a technique in which a back electrode is formed in a shape having unevenness (texture structure) of a size substantially the same as light wavelength, for scattering light. This lengthens the optical path in a semiconductor layer by scattering light at long wavelengths which is not absorbed by the semiconductor layer, and increases a short-circuit photoelectric current by improving the long wavelength sensitivity of a photovoltaic device, thereby improving the efficiency of photoelectric conversion.
Zinc oxide has higher resistance to a plasma than tin oxide and indium oxide, and is not reduced by hydrogen even in a plasma containing hydrogen. Therefore, in forming a semiconductor layer comprising amorphous silicon on a transparent conductive layer by a plasma CVD process, a zinc oxide thin film is positively used as the transparent conductive layer.
Japanese Patent Laid-Open No. 60-84888 (Energy Conversion Devices) discloses a technique in which a transparent conductive layer is interposed between a back electrode and a semiconductor layer to decrease a current flowing in a defect region of the semiconductor layer.
On the other hand, as seen in 24th IEEE First WCPEC; Dec. 5-9, 1994, p. 254 xe2x80x9cEFFECTS OF Cd-FREE BUFFER LAYER FOR CuInSe2 THIN-FILM SOLAR CELLSxe2x80x9d; T. Nii, H. Takeshita, a technique using a zinc oxide thin film as a n-type window layer of copper-indium-selenium (Cu-In-Se2: CIS), copper-(indium, gallium)-selenium (Cu-(InGa)-Se2; SIGS), or the like has recently been disclosed.
As described above, a zinc oxide thin film is preferably used as the light confinement layer having a texture structure surface.
Although conventional known methods of producing a zinc oxide thin film include a vacuum deposition method, a sputtering method, an ion plating method, and a CVD method, all methods require an expensive vacuum apparatus, and an expensive vaporization source. Also the light confinement effect at wavelengths of 600 to 1000 nm is insufficient.
Other known methods include wet methods such as spray pyrolysis, a sol-gel method, a dipping method, and the like. However, in these wet methods, a substrate must be heated to about 300 to 800xc2x0 C., and thus substrates which can be used are limited. Also, zinc hydroxide is contained in the thin film together with zinc oxide, and it is thus difficult to form a pure zinc oxide thin film.
As seen in Japanese Patent Laid-Open No. 8-217443, Journal of Electrochemical Soc. Vol. 143, No. 3 xe2x80x9cElectrolyte Optimization for Cathodic Growth of Zinc Oxide Filmsxe2x80x9d; Masanobu Izaki, Takashi Omi, it has recently been reported that a counter electrode is immersed in an aqueous solution of zinc nitrate, and a current is passed to electrochemically deposit a transparent zinc oxide thin film.
Also a technique of forming a zinc oxide thin film by a liquid phase deposition method has been reported in xe2x80x9cFormation of ZnO Film by Electrolysis in Aqueous Solutionxe2x80x9d, (Autumn, 1995), 65th Applied Physics Society, p. 410.
In these methods, since an expensive vacuum apparatus and expensive target are unnecessary, the production cost of a zinc oxide thin film can significantly be decreased. Also a thin film can be deposited on a large substrate, and the methods are thus advantageous for large photovoltaic devices such as solar-cells. However, the electrochemical deposition methods have the following problems:
(1) Particularly, an increase in current density or concentration of the solution causes the problem of easily producing abnormal growth of a needle-like, spherical, resin-like shape of the micron order or more on the deposited thin film. The use of such a zinc oxide thin film as a part of a photovoltaic device possibly causes the abnormal growth to induce a shunt path in the photovoltaic device.
(2) In the zinc oxide thin film formed by one of the above methods, unevenness easily occurs in the grain sizes of zinc oxide crystals, and particularly, the problem of unevenness occurs in the case of a large area.
(3) The zinc oxide thin film formed by one of the above methods has lower adhesion to a substrate than the vacuum deposition method using resistance heating, an electron beam or the like, the sputtering method, an ion plating method, and the CVD method.
Conventionally, only smooth thin films are formed by the electrochemical deposition methods, and a method of electrochemically depositing a zinc oxide thin film having unevenness for the light confinement effect has not been established yet.
Accordingly, it is an object of the present invention to stabilize formation of a zinc oxide thin film by electrodeposition, and provide a method of forming a zinc oxide thin film having excellent adhesion to a substrate. Particularly, a zinc oxide thin film suitable for application to a light confinement layer of a photovoltaic device is formed.
In order to achieve the object, the present invention provides the following methods of producing a zinc oxide thin film and photovoltaic devices formed by using the producing methods.
(1-1) A method of producing a zinc oxide thin film comprising passing a current between a conductive substrate immersed in an aqueous solution containing at least zinc ions, ammonium ions, and zinc ammonia complex ions, and an electrode as an anode immersed in the aqueous solution to form a zinc oxide thin film on the conductive substrate. This producing method is capable of electrochemically forming a zinc oxide layer and forming an uneven surface which can sufficiently exhibit light confinement or light scattering for light at wavelength of 600 to 1000 nm. Therefore, it is possible to produce a photovoltaic device having high quality and low power cost. Particularly, the production cost of the zinc oxide layer can be decreased to about 1/100 of the sputtering method.
(1-2) In the method of producing a zinc oxide thin film, the conductive substrate comprises a transparent conductive layer deposited on a support. This producing method is capable of relatively easily and uniformly forming an initial film of zinc oxide, and efficiently forming the zinc oxide layer. Also, in a photovoltaic device, the zinc oxide film can be electrochemically formed on the conductive substrate by protecting a high-reflectance metal previously formed on the conductive substrate having an important function.
(1-3) In the method of producing a zinc oxide thin film, the hydrogen ion concentration (pH) of the aqueous solution for forming the zinc oxide thin film is controlled in the range of 8 to 12.5. This producing method is capable of relatively stably supplying complex ions such as the zinc ammonia complex ions or the like by keeping the solution for forming the zinc oxide thin film alkali, and efficiently forming the zinc oxide layer. Also, since the solution for forming the zinc oxide thin film is kept alkali, a wide range of substrates having no need for acid resistance can be used.
(1-4) In the method of producing a zinc oxide thin film, the hydrogen ion concentration pH of the aqueous solution near the uppermost surface where the zinc oxide thin film is formed is controlled in the range of 6 to 8. Since the hydrogen ion concentration pH of the aqueous solution near the uppermost surface where the zinc oxide thin film is formed is locally controlled in the range of 6 to 8, the dense zinc oxide thin film can be continuously formed. At the same time, the hydrogen ion concentration of the aqueous solution in the bulk region is preferably in the range described in (1-3).
(1-5) In the method of producing a zinc oxide thin film, the aqueous solution for forming a zinc oxide thin film contains a hydrocarbon. This producing method can significantly suppress abnormal growth in the zinc oxide layer, and thus further improve yield. The uniformity of the zinc oxide layer can further be improved.
(1-6) A method of producing a photovoltaic device comprising the step of forming the zinc oxide thin film. This forming step is capable of electrochemically forming the zinc oxide layer and forming an uneven surface which can sufficiently exhibit light confinement or light scattering for light at a wavelength of 600 to 1000 nm. Therefore, it is possible to produce a photovoltaic device having high quality and low power cost. Particularly, the production cost of the zinc oxide layer can be decreased to about 1/100 of the sputtering method.
(2-1) A method of producing a zinc oxide thin film comprising passing a current between a conductive substrate immersed in an aqueous solution containing at least zinc ions, hydrogenzincate ions (HZnO2) and zincate ions (ZnO22xe2x88x92) and an electrode as a cathode immersed in the aqueous solution to form a zinc oxide thin film on the conductive substrate. This producing method is capable of electrochemically forming a zinc oxide layer and forming an uneven surface which can sufficiently exhibit light confinement or light scattering for light at wavelength of 600 to 1000 nm. Also, since the zinc oxide thin film is formed on the conductive substrate on the anode side, the zinc oxide thin film contains no zinc metal. Therefore, it is possible to produce a photovoltaic device having high quality and low power cost. Particularly, the production cost of the zinc oxide layer can be decreased to about 1/100 of the sputtering method.
(2-2) In the method of producing a zinc oxide thin film, the conductive substrate comprises a transparent conductive layer deposited on a support. This producing method is capable of relatively easily and uniformly forming an initial film of zinc oxide, and efficiently forming the zinc oxide layer. Also, in a photovoltaic device, the zinc oxide film can be electrochemically formed on the conductive substrate by protecting a high-reflectance metal previously formed on the conductive substrate having an important function.
(2-3) In the method of producing a zinc oxide thin film, the hydrogen ion concentration (pH) of the aqueous solution for forming the zinc oxide thin film is controlled in the range of 8 to 12.5. This producing method is capable of relatively stably supplying ions such as the hydrogenzincate ions, zincate ions and the like by keeping the solution for forming the zinc oxide thin film alkali, and efficiently forming the zinc oxide layer. Also, since the solution for forming the zinc oxide thin film is kept alkali, a wide range of substrates having no need for acid resistance can be used.
(2-4) In the method of producing a zinc oxide thin film, the hydrogen ion concentration pH of the aqueous solution near the uppermost surface where the zinc oxide thin film is formed is controlled in the range of 6 to 8. Since the hydrogen ion concentration pH of the aqueous solution near the uppermost surface where the zinc oxide thin film is formed is locally controlled in the range of 6 to 8, the dense zinc oxide thin film can be continuously formed. At the same time, the hydrogen ion concentration of the aqueous solution in the bulk region is preferably in the range described in (2-3).
(2-5) In the method of producing a zinc oxide thin film, the aqueous solution for forming a zinc oxide thin film contains a hydrocarbon. This producing method can significantly suppress abnormal growth in the zinc oxide layer, and thus further improve yield. The uniformity of the zinc oxide layer can further be improved.
(2-6) A method of producing a photovoltaic device comprising the step of forming the zinc oxide thin film. This forming step is capable of electrochemically forming the zinc oxide layer and forming an uneven surface which can sufficiently exhibit light confinement or light scattering for light at a wavelength of 600 to 1000 nm. Therefore, it is possible to produce a photovoltaic device having high quality and low power cost. Particularly, the production cost of the zinc oxide layer can be decreased to about 1/100 of the sputtering method.
(3-1) A method of producing a zinc oxide thin film comprising passing a current between a conductive substrate immersed in an aqueous solution containing at least carboxylic acid ions and zinc ions, and an electrode as an anode immersed in the aqueous solution to form a zinc oxide thin film on the conductive substrate. This producing method is capable of forming the zinc oxide film having excellent optical properties at low material cost without the need for a large-scale apparatus.
(3-2) In the method of producing a zinc oxide thin film, the aqueous solution is an aqueous solution of zinc acetate. This producing method is capable of forming the zinc oxide film having excellent optical properties at low material cost without the need for a large-scale apparatus.
(3-3) In the method of producing a zinc oxide thin film, the aqueous solution is an aqueous solution of a zinc formate. This producing method is capable of forming the zinc oxide film having excellent optical properties at low material cost without the need for a large-scale apparatus.
(3-4) In the method of producing a zinc oxide thin film, the conductive substrate comprises a transparent conductive layer deposited on a support. This producing method is capable of forming the zinc oxide thin film having less abnormal growth and excellent uniformity.
(3-5) In the method of producing a zinc oxide thin film, the hydrogen ion concentration pH of the aqueous solution for depositing the zinc oxide thin film is controlled in the range of 3.5 to 5.5. This producing method is capable of forming the zinc oxide thin film having less abnormal growth and excellent uniformity.
(3-6) A method of producing a photovoltaic device comprising the step of forming the zinc oxide thin film. This producing method is capable of stably forming the high-performance device at low cost.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.